text:
Is a jet engine's shaft neutral in the direction of thrust?
I think I read somewhere that the forces on a jet engine shaft is neutral because the forces at the turbine and compressor cancel out. This means that the axial bearings don't have force on it in direction of thrust!?
Is this true? In that case I think the thrust will be zero because there's noting against which the pressure can push to produce thrust.

Not necessarily. Some jets doesn't use a straight nozzle so the thrust vector is not aligned with the shaft, with the extreme cases like VTOL airplanes (e.g. F-35) that the thrust is perpendicular to the shaft. Even when this is the case, the pressure behind the fan and compressor equalizes itself. "Cancel" here means there's no imbalance, not no force.

Thrust is produced in the compressor; as its blades push the air backwards to compress it, a reaction force pushes the blades forwards. These blades are fixed to the shaft. The shaft has a flange on it which pushes against a similar face on the engine mounting, in an arrangement known as a thrust bearing. This in turn transmits the thrust to the airframe.
A turbofan has extra big blades which push more air backwards, adding to the thrust.
The turbine blades draw work from the exhaust flow which slows it down, so they push against the main thrust and in a simple single-spool design can even require the thrust bearing to face backwards.
Heating the compressed air in between is what makes the thing work at all. Also, the acceleration of air out the back of the combustion chamber creates a thrust reaction against its forward wall. For a pure turbojet, and especially for a centrifugal compressor, this will be greater than the compressor thrust. It might not be as much as one might suppose because the chamber is relatively small.
For supersonic flight, the air intake acts as a shock compressor to generate most of the thrust, while the exhaust nozzle can be made to generate more by expanding high-pressure exhaust against a divergent nozzle. At high enough speeds the spinning bits just get in the way.

@ guy inchbald,link: https://www.youtube.com/watch?time_continue=171&v=JmqEU2C0Vu4&feature=emb_logo      he said turbine has more force backwards than compressor has forward and so thrust at thrust bearing is backwards!!!??
this cant be true..

If that was true, a bucket reverser in the tailpipe of a turbojet would have little effect wouldn't it.  Or a vectored thrust nozzle.  The F-35 wouldn't be able to hover, because there is all that forward thrust coming from its compressor deep in the engine so what's the point of pointing the tail pipe down?  Most of the trust is happening at the exhaust nozzle, which is why you can point it in different directions and get the thrust axis you want by where you point it.  Even the Harrier is the doing the same thing with its fan flow directed out nozzles.

Also if this was true, a centrifugal compressor turbo jet wouldn't make much thrust at all would it, with the spinny wheel thingy flinging air out.  No "lift" there at all.

How does a ramjet work with no compressor?

"spinny wheel thingy" always gets a +1 from me, @JohnK!

I was thinking "what the heck is that word... imp-something... ah screw it, I'll go with spinny wheel" lol

@guy inchbald ,look at my picture, rearward thrust from turbine is grather than forward thrust from compressor,so shaft is pushing on thrust bearing in backward direction!!!

I made some corrections, I was focusing too much on supersonic flight.

@OrganicMarble How a ramjet works is a different question and has probably been asked and answered already. It's all in the addition of heat energy and asymmetric profiling of the duct.

In a pure turbojet pretty much all the thrust is coming from the expansion of gasses from heat released by the burning of fuel in the compressed air stream.  The acceleration of the gasses out the burner can and on to the tailpipe nozzle is the primary action/reaction going on that makes thrust (and mostly in the convergent nozzle of the tailpipe).
The thrust force itself is not "pushing" on anything.  It's gas that is expanding and has only one way to go, so it accelerates that way and you have your Newtonian action/reaction.  Like letting go of a balloon. The thrust comes from the acceleration of air, being compressed by the balloon's rubber, out the ballon's mouth after you let it go.
Thrust loads from the compressor are largely offset by drag loads on the turbine, so the spool is sort of trying to stretch itself all the time, although there may be a net forward thrust component to the extent that the compressor's thrust load exceeds the turbine's drag load, and the ball bearings will be designed to handle whatever axial load there is.  I've never found any source that says that any thrust surplus produced by the compressor itself is significant in the total thrust of the engine.
Also think of a turbojet with a centrifugal compressor. No blades making forward lift, just curvy channels flinging air out.  How could you get any thrust from that? The centrifugal turbojet is getting its push from the same place as an axial one; between the turbine and exhaust nozzle.
On a turbofan, a significant part of the energy in the exhaust flow is extracted by a separate turbine to make torque to drive the fan.  A turbofan is more or less a free-turbine turboprop with an un-geared, fixed pitch propeller with oodles of blades.  There's a bit of thrust out the tailpipe from expanding gas energy surplus to what was extracted by the fan's turbine, and perhaps a bit of surplus thrust load from the core compressor, but most is from the fan, and in that case the fan's forward bearings will be designed to handle most of the engine's thrust capability.

action-reaction dont explain how fluid(air) exert force to the jet engine at all.air can exert force to the object only through pressure ,so integration of pressure at blades and walls of jet must show force,if you have two fans on same shaft ,once pull to the left one to the right with same force(shaft is neutral) now pressure integration is zero ,so thrust is zero...so if you said that shaft has almost zero load ,pressure must push aginast some walls in jet engine ,maybe at nozzle walls..

Of course compressors produce thrust. I have no books about jet engines as such, but Kermode (Mechanics of Flight) notes that the turbine produces drag but the turbine-plus-compressor is what produces thrust at zero airspeed. Think; each compressor blade works as a airfoil by "lifting" forwards to push air backwards. Think too that if that did not work, then a bypass turbofan would not work either; compressors, fans and propellors are all spinning wings doing much the same job. Also, why else would the shaft be in tension yet need a thrust bearing at the front?

@guy inchbald,are talking to me or to John K,because i have oppinion same like you...

Of course the compressor produces a thrust load, but much of it is offset by the turbine, and whatever net thrust force in the spool remains is small in relation to the acceleration of the mass flow generated by heat expansion from fuel.  In a pure turbojet most of the thrust is generated at the exhaust nozzle which converges to force the mass to accelerate further.  A turbofan's, or a turboprop's, or a turboshaft's, torque is coming from all the energy still present AFTER the compressor's turbine, which is the majority of the energy produced by the engine.

do you understand that air can exert force on object only through pressure difference?

Sure but the thrust force of an engine is coming from the acceleration of a mass through a restriction. It's the velocity not the pressure.  When you blow up a party balloon and let it go, there is a pressure differential acting in all directions around the sphere of the balloon no?  It goes in one direction because the balloon's throat directs the air out a small opening, forcing to accelerate to high velocity. The pressure is the energy potential.  The thrust is from the acceleration of the gas out a restricted opening.

@john K ,you are completly wrong,nozzel produce backward thrust,look at my original post,i add picture...(and ballon is going forward because where is open end there is no wall where will internal pressure push agianst,so we have pressure imbalance which drive ballon forward..)

So then, how do you get thrust with a centrifugal compressor?

Also in that diagram, assuming it's an accurate representation, the aft thrust load on the turbine is double the forward thrust load from the compressor and the effective thrust is coming from the mass flow accelerating out the burner can as it expands from the heat added to it.  So the net axial bearing load is massively AFT.

I think some things are getting muddles in this discussion. Yes, the nozzle is a key part in "generating" thrust, but in itself, the nozzle does not 'push', but rather 'pulls' on the engine casing (from a 'force' perspective, it is just there to trade flow rate for higher working pressure). In the end, the actual thrust force acts on (any of) the forward-facing walls of the flow path. (1/2)

So in the case of a centrifugal compressor, the force acts on the casing that diverts the flow backward, so we only have turbine drag. On the other hand one could imagine an axial compressor and centrifugal turbine, in which case the load on the thrust bearings is inverted. The only logical conclusion is that the net force on the thrust bearings is dependent on the design of the compressor and turbine. (2/2)

That's why I added the qualifier "to the extent that the thrust load exceeds the drag load".  I think that the vast majority of the "reaction force" of a turbojet originates in the region of the nozzle and if the nozzle is straight, that reaction force is distributed through the structure of the engine forward.But if you have an exhaust nozzle pointing down, like the F-35B's, nearly all of the thrust surplus to fans' power requirements is acting on the top of the nozzle duct, vertically. That's why I say that most of the thrust force has its origin in the flow's acceleration toward the nozzle.

The forces almost cancel, but not quite.  You might have 10,000 lbf forward and 9,500 lbf aft, for a net of 500 lbf forward.  This is intentional.  If the thrust on the bearings is too low, that's actually bad.  It can lead to a phenomena called "skidding".  Ball bearings are designed to work with some finite amount of axial load. Too much is bad, because it leads to increased stress, but too little is also bad.  Engine designers can tweak the rotor thrust by adjusting various parts of the design, such as seal diameters (the part where the rotating shaft meets the stator).   There will be some high pressure seals in the turbine, with maybe 750 psi on one side and 1000 psi on the other.  If you increase the seal diameter a little, that adds some rotor thrust, and if you decrease the seal diameter is decreases it.  The diameter will be adjusted to get the desired rotor thrust.
Edit: To clarify... I'm thinking here of the core rotor on a turbofan.  I guess if you are thinking of a pure turbojet,  then this might not apply

if forces cancle then thrust is zero.did you see my picture with load distrubution in jet turbine?

@AeronauticFreek: There is enough structure left besides the spinning blades against which the air pushes in order to create thrust. The diffusor is one such place and the nozzle another one. In supersonic engines the intake produces most of the thrust.

@Peter Kampf, does that mean that all walls inside jet engine which produce forward thrust,must "facing" forward even a little bit,because pressure act only prepedicular to the surface?

if the forces on the rotor cancel, then the net force on rotor is zero.  the net force on the entire engine is not zero.  The rotor is only part of the engine.

@AeronauticFreek: Yes, like a rocket chamber.

@Peter Kampf,ok that make sense.so in turbo-jet forces almost cancle,but does  turbo-fan has huge thrust at shaft in forward direction or it is almost  neutral too?

Most of thrust reaction force originates in the tailpipe leading to the nozzle and the net direction of the thrust force is a function of the orientation of the nozzle.The F-35B is able to direct half of its dry thrust down out the tailpipe (other half of the engine's power potential is consumed by forward lift fans) for hovering. If there was significant reaction force further forward in the engine, you would have to tilt the entire engine.  When level, the reaction force acts linearly by distributed pressures forward, but when rotated down it's acting on the upper wall of the tail pipe duct.

@DanielK,so turbo fan is almost neutral and what is with turbojet? why you think that he dont has almost neutral rotor?

The theory is the same for a turbojet, as mentioned in John K's answer, so very possible for rotor thrust to be zero.  However, I don't have any first hand experience with turbojets.  Turbofans, I have experimentally measured the ball bearing axial thrust on the core rotor and I can say first hand it is possible to be zero.  Turbojet I've never worked with.

@DanielK,yes it is very non intuitive that this huge fan cancle axial load with turbine!my picture of turbojet and video say that thurst bearing hold backward thrust,which is even more non intuitive..so all the thrust interact with static parts, stator blades and difuser etc


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How calculate water rocket thrust?
both nozzle is 2cm diameter
one "rocket" has air pump which hold constant 5 bar
second "rocket" it has water inside and also air pump which hold cosntant 5 bar,(water column is only 30cm haight,so hydrostatic pressure is neglibile,dont take into account)
rocket diameter 25cm
How much is thrust in air and water case?(load cell measure thrust)

NOTE:
FLUID IN TANK HAS SOME VELOCITY,SO PRESSURE IN TANK IS NOT 5BAR IN EVERY POINT!

this goes to Physics SE

nobody want to answerd there

you got downvoted?

i dont know ,maybe nobody knows answer so no one answerd..so i delete my question

I would have just let the question sit. Somebody might dig it up.

Before anyone asks, OP doesn't have enough rep on physics SE to offer a bounty.

@Abdullah it is still not a good idea to re-ask a question on a different stack

@Federico wdym?

@Abdullah if it gets closed on the site where it is most on topic, the best thing to do is try to fix the question there, not to ask it where it is dubiously on topic

@Federico huh? It's very relevant on Physics, and dubiously relevant here. Anyway, he said that he deleted it, not that it was closed.

@Abdullah yes, that's what I meant. I didn't know OP self-deleted without it being closed. nevertheless they should not have moved it here.

@Federico agreed. As I said earlier, if I were the OP, I would just have waited, not given up.

One can calculate this from first principles. Consider a water packet with volume $V$ going across a pressure differential $p$. Kinetic energy of this particle is  $$E=pV.$$ Water is incompressible so the mass of the packet $m=\rho V$. Since $E=\dfrac{1}{2}mv^2$ we can calculate $$v=\sqrt{\dfrac{2E}{m}}.$$ The momentum of our water packet $q=mv$. Filling in all the equations so far, we get $$q=V\sqrt{2p\rho}.$$ Thrust is momentum per unit of time, so we get $$F=Q\sqrt{2p\rho}$$ with Q the water flow.
This leads to the logical conclusion we can double the thrust by adding twice as many nozzles (but we'll run out of water twice as fast). For the air case, simply replace the density of water $\rho=1000\mathrm{kg/m^3}$ with that of air. For a given volumetric flow, thrust will be a lot less, as expected.
A simpler way to look at things may be a force balance. The internal pressure $p$ is balanced everywhere except at the nozzle. If the nozzle has a cross section $A$, we get $$F=pA.$$ This is independent of the fluid used, but looking at our previous equation we see that for a less dense fluid we will need a higher flow rate.
Note that all of the energy from your setup comes from your fictional ideal air pump, so while in your question your can just increase the pressure to get better thrust, the reality is that you will increase the energy requirements faster than your thrust will grow (because simply the linear vs quadratic relation for momentum and trust).
Addendum: I did a quick check to see what the exit velocity of the air should be and it's over two times the speed of sound at sea level. So unless you have a high tech convergent-divergent nozzle you're better off using water

This is only true when the water/air is being expelled into vacuum.

@JZYL p is the pressure differential here and is 4 bar.

@JZYL The question also didn't specify if the pressure was 5 bar absolute or 5 bar above ambient.

@Sanchises,pressure is 6 bar absolute,you and Peter K. show thrusts is not same its dependt on fluid density,JZYL show pressure is equal  and not dependt on fluid  density, who is correct??

@Aeronautic I actually just amended my answer upon seeing the other answer. For a constant nozzle, thrust is the same, but for a constant flow rate, thrust increases with density. The answers are functionally identical as far I can tell.

@Aeronautic Exactly! The reason we use water is because we need less volume (because our bottle rocket is only so big) and because it is more energy efficient (so you either need a smaller pump or less compressed air to store the energy)

@Sanchises,problem is that VELOCITY of fluid in tank is NOT ZERO ,so 5bar is not in every place in tank...

@Aeronautic Sure, I indeed assumed 100% efficiency, so quasi-static behaviour. I think with a well-designed nozzle you should be able to get pretty close as long as the exit velocity of the fluid is subsonic.

@Sanchises,if take into account that fulid in tank has some velocity,what tank will produce greater thrust with air or water?

@Aeronautic You're now in the territory of engineering, not Q&A. So my best answer is "it depends" but I think water will be better because despite the higher viscosity the velocity is much lower so the overall losses less.

@Sanchises I see your answer specifies pressure differential. Good to see both methods arrive at the same answer!

@Sanchises,your thrust formula from pressure imbalance is F=pxA....5bar x 3.14cm2=154N,,pressure in nozzle is not 5bar,bacause fluid has high velocity,so pressure here is less than 5bar..

@AeronauticFreek That's not the right thinking. If hydrostatic pressure is negligible and the surface pressure is 5bar, then that's the pressure throughout the tank, regardless of how fast the water/air is moving inside the tank. If the speed inside the tank is much smaller than the nozzle speed, then its effect is negligible (1/2Ve^2 - 1/2Vi^2 ~= 1/2*Ve^2) in Bernoulli's energy balance.

@JZYL,here i am talking about pressure in nozzle not in tank,velocity of fluid in nozzle is very high,so i think pressure in nozzle is less then 5bar. isnt it?

@Aeronautic Yes, that reduction in pressure is exactly what accelerates the fluid. Over the span of the nozzle pressure will reduce from 5 bar to 0 bar above ambient.

@Sanchises, ok so thrust in both case  is 5bar x 3.14cm2=154N ?

Try a third rocket with mercury.  Instead of a load cell, just let all 3 run down a track.

@RobertDiGiovanni I tend to prefer backyard experiments that do not come with the risk of mercury poisoning. But perhaps that's just me...

@Sanchises Yes, yes, increasing pressure (within safe limits) may be better.  There are water rocket videos using plastic soda bottles.  Makes me think it is more the momentum action/reaction than the internal/ambient pressure differential that makes it go.  Would look into nozzle length and dimensions as well!

@Robert Can you maybe take an introductory course in dynamics sometime? Your statement makes no sense at all. This is Newton's laws in their most basic form and I shouldn't still have to explain them. By momentum conservation for the bottle rocket to move one way something else has to move the other way (3rd law) and all this moving is done by forces (2nd law). Neither law is "more" responsible.

@Sanchises humor aside, I believe there is a firm understanding of m and v here without an "introductory course in dynamics". I was referring to the other answer, which held that type of fluid does not matter.  Upvoted yours, now, would mercury create more thrust?

@Sanchises, onece agian, is thrust in both case 5bar x 3.14cm2=154N ?

@Aeronautic I'm here to explain things, not do the maths for you. If you want anything clarified feel free to ask.

@Sanchises, i just use formula from your post  F=p(internal prressure 5bar) x A(nozzle area 3.14cm2)...now with this formula(basic pressure equation) it seems very easy to calcuate thrust

@Aeronautic Who said it had to be hard?

@Sanchises,I read that thrust is F=mass flow x V + pressure x A,but your formula dont have part " mass flow x V" ..why is that?

@Aeronauti Where did you read that?

@Sanchises,https://www.grc.nasa.gov/www/k-12/airplane/rockth.html

Let us continue this discussion in chat.

Consider control volume around the whole apparatus, and apply the momentum balance using the steady-state Euler's equations in the integral form:
$$\vec{F}_{body} = \oint_S(\rho\vec{V} \cdot d\vec{S})\vec{V} + \oint_Spd\vec{S}$$
If the nozzle is aimed parallel to the ground, then it simplifies to the 1D thrust equation, noting that the exit pressure is equal to the ambient pressure ($p_{atm}$):
$$T = \rho V^2 A_e \tag{1}$$
where $T$ is thrust, $\rho$ is the density of the flow right before the nozzle exit, $V$ is its exit speed, and $A_e$ is the nozzle frontal area.
We are going to assume there is zero loss in this whole endeavour, so the flow is isentropic everywhere, which means we can apply Bernoulli's to derive the exit speed (noting that inside the tank, the fluid velocity is zero, assuming the nozzle is small compared to the total volume):
$$V=\sqrt{\frac{2(p-p_{atm})}{\rho}} \tag{2}$$
where $p$ is the pressure of the fluid inside the tank.
Now substitute (2) into (1), simplify, and we have 1:
$$T=2(p-p_{atm})A_e=Q\sqrt{2\rho(p-p_{atm})} \tag{3}$$
where $Q=VA_e$ is the volumetric flow rate.
So the thrust is independent of what kind of fluid you use! However, it should be cautioned that this does not mean that a compressed air rocket will receive the same $\Delta V$ as the water one when their tanks are emptied. More factors, such as empty weight and tank volume, must be taken into account. For instance, the water rocket will have much lower volumetric flow rate than the air one, giving it a longer "burn" time.

1 Note the conclusion reached is identical to @sanchises' answer, despite the slightly different approaches.

Isn't thrust proportional to $\sqrt{\rho}$? That would give the water version 28 times the thrust of the air version.

@JZYL ,problem is that VELOCITY of fluid in tank  is NOT ZERO ,so  5bar is not in every place in tank...

@PeterKämpf,  if we integrate pressure around all rocket walls we will get thrust,and if in both case is 5bar inside,doesnt metter which fluid is inside...isnt it?

@PeterKampf Not if I specify all variables in terms of pressures. However, the water rocket will have a longer "burn" time, since its flow rate is lower, with the same thrust.

@AeronauticFreek The velocity inside the tank is much smaller than the nozzle velocity (5cm vs 30cm), so this assumption holds.

@JZYL your mathematical derivations seem sound, but why would you think the water comes out much slower?.  You are correct that the water rocket generates more thrust, and it is possible that viscosity may make the expulsion of water slightly slower, but it may be that higher density (mass per volume) carries the day.  I was wondering if a longer nozzle would increase thrust for water, giving a greater distance to accelerate.

Your calcs may be correct if it is just a hole in the container.  Garden hoses can be interesting.

@RobertDiGiovanni Because math told me so?

So, lengthen the tube and allow water to accelerate before discharge.  Might make math easier, discounting friction loss.

@RobertDiGiovanni If there is zero loss, then all of the venturi effect has already happened through the initial opening. Elongating the opening won't achieve anything.

@JZYL theoretically, but a converging nozzle with water might be worth a try.  The water system is a bit like an internal combustion engine cylinder, pushing on a liquid instead of a solid piston.  If the water is accelerated as much as possible, it will generate more thrust.  (Converging/diverging will optimize the supersonic air).  I believe you are right, the water stream will be slower.  Amazing how the humble soda bottle makes a fairly good converging nozzle.  Shame they closed this.

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Am I required to report an incident that isn't specifically mentioned in 49 CFR 830?
I have read an understand NTSB Part 830, and know that IMMEDIATE notification to the NTSB is required for aircraft accidents and the listed serious incidents. My question is, is there a requirement to notify the NTSB or FAA of an incident that is not listed in part 830? This is not a question about accident reporting, but specifically incident reporting for incidents that are not listed in the regulation.
Possible scenario: While landing at an uncontrolled field, the airplane loses directional control and departs the runway into the grass and the airplane did not receive substantial damage nor was there a serious injury or death.

Welcome to aviation.SE! I believe your question has already been answered, but if not then please feel free to edit it to add more details about what you need that isn't covered in the other question. You might also find the [tour] is helpful if you're new to the site.

The other question doesn't reference notification for incidents that are not listed in the regulations, which is what this question is about.

Thanks for the update! I'm just not sure I understand how you could be required to report something if there's no regulation. Your insurer or flying club might want to know, of course. I'm not trying to be difficult, I just don't know how to answer the question of how to report something when there's no requirement to report it. Maybe this question helps?

Maybe there is a secret regulation.

There are a couple of part 91 regulations that require 'general' incident reporting, but only if the FAA or ATC requests it.
14 CFR 91.3(c) says that if a pilot exercised emergency authority to deviate from part 91 rules then the FAA can ask for a report:

(b) In an in-flight emergency requiring immediate action, the pilot in
command may deviate from any rule of this part to the extent required
to meet that emergency.
(c) Each pilot in command who deviates from a rule under paragraph (b)
of this section shall, upon the request of the Administrator, send a
written report of that deviation to the Administrator.

91.123(d) says that ATC can ask for a report if they gave a pilot emergency priority, even if the pilot didn't deviate from any rules:

(d) Each pilot in command who (though not deviating from a rule of
this subpart) is given priority by ATC in an emergency, shall submit a
detailed report of that emergency within 48 hours to the manager of
that ATC facility, if requested by ATC.

There are also regulations for charters (135.415) and airlines (121.703) that have a long list of "service difficulties" that must be reported, including in-flight fires, brake failures and others. And there are likely non-FAA regulations on things like HAZMAT incidents.

The short answer is no. You do not have to report it to the NTSB or FAA. But, it depends on the circumstances and amount of damage. Though, you can and probably should file a NASA report, anyway. This will contribute to statistical analysis of accidents for future accident prevention and safety.

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text:
Do aircraft at cruise bob and/or skip?
Recently I started flying a model aircraft, admittedly a kindness for a motor stuck onto a piece of foam, and have noticed that it either bobs, for lack of a better term, like something afloat on water might when depressed or that it skips, for lack of a better term, like a stone thrown across the surface of a pond might.
In the first case the motorized foam is at cruise at constant speed at some height when I force it into a dive it seems to "bob" back up to it's original height when I stop giving it the input to dive. I suspect I'm observing a little Newtonian mechanics here trading potential energy for kinetic energy during the dive and that this gets converted back as it "bobs" up again.
In the second case when the motorized foam is at cruise at constant speed at some height it tends to undulate. This is exacerbated if I increase the angle of attack(?) such that in the extreme, where it stalls, the motorized foam goes into a cycloidal "swoop". Each "swoop" starts with a stall where it'll "flip over" and dive down and gaining speed as it does so, the gain in speed results in the nose "picking up" resulting in a "swoop" back into a near vertical "climb", at roughly the same height as when it first stalled it'll stall again and repeat the process.
Do larger scale aircraft, that is anything with at least a human on board, behave in this manner as well? In the case of "bobbing" do pilots use this in any way to their advantage e.g. does it help them maintain altitude or is it helpful in some way during dire situations? (My imagination can't help but wonder if there was ever a pilot who forgot to throttle down on decent, intending to land, who found they simply "bobbed" back up to their pre-decent altitude?)
In the case of "skipping" I can't imagine passengers being very agreeable to such a motion, especially not the stalling swoop variant, but I'm fairly certain I've felt a plane undulating slightly on a flight before. Do pilots have to constantly fight such undulations or do they reduces/eliminate this through proper trim?
Note:
I'm not talking about wing in ground effect, I observe this when the motorized foam is at roughly tree top height. The veldt I fly in comprises of knee high grass so I doubt I'll observe this until I fly over a lawn or something.
Note:
If it's relevant motorized foam is shaped like a flying wing; in case aircraft with tails behave differently.
Note:
I've certainly observed that the wing tends to "pitch up" in proportion to its speed above a certain speed; this is most noticeable in the cycloidal "swooping". Initially I thought this was a CG thing or that the motor was off center and generated a moment force but I see this is already discussed. It's more likely my trim is off.
Update

Update :
I phugoid it out, I believe, stuck a coin under the battery (rotating it made it poke out like an Adam’s apple) and I trimmed the trim to something slim. Did the glide test and she stays level as a lake wafting gracefully from the skies like Dumbo with his trunk out, tail tucked and his C.G. Is under his chin ! Thanks all for the helpful hints and the epoxy related non-confessions. (When the coin is worth more then the model I’ll fork out for an upgrade. Till then I’ll wait for the back of this cold front)

+1 for "a model aircraft, admittedly a kindness for a motor stuck onto a piece of foam" and "the motorized foam"! Nice! :)

"in case aircraft with tails behave differently." They do. We put the tails there for a reason. :) In particular, they're quite useful for stability.

@RobertDiGiovanni I'll happily do so in the morning ... no judgements on the rivers of epoxy holding it together though ;P in the interim I will include an image of a pristine variant.

Note that the "rivers of epoxy holding it together" may well have thrown off the CG. Also, the duck tape "trim tab" on the near wing probably isn't helping much either.

"...rivers of epoxy..." - when I was learning to fly R/C airplanes many years ago the standard beginners aircraft at our club was a foam model of a Piper Cub because it was A) easy for a beginner to put together, and B) could be repaired on the flight line by the appropriate application of five-minute epoxy. So a beginner could take his plane up (usually on a buddy cord with a more experienced pilot), fly for a while, crash his plane, gather up the pieces, smoke a cigarette/calm down, fix his plane, and be ready for another flight within 15 minutes or so. Don't ask me how I know... :-)

The air mechanics "epoxy tears" are super handy, not that you'd know of course, given that the odd passer/family friend by who unwittingly shows even the slightest interest is given the controls and a crash course. Teenagers seem to hit the dirt uttering "... but I play video games !". The septuagenarians try maintain an air of decorum and a facade of nonchalance towards "toys" as they insist on another turn and laugh manically when they collide with the trees. Women, curiously, feign a timidness on the first throw then calmly and determinedly get airborne before any "man" can.

@FreeMan the duct tape "trim tab" is more to stop the white duct tape "wing tip guards" stay in place, similarly the duct tape "winglet guards" hold the duct tape "skids"  in place during brick pavement and dirt road landings and power line "touch 'n go's". Thanks though :D I'll keep that in mind while getting thrifty with the pennies.The sticky faces of most such modifications are now all coated in a thin veneer of and mother natures' "dandruff" and in need of replacement.

It sounds very much what you're experiencing is a Phugoid cycle

Image License info
To put simply, your aircraft is not trimmed correctly. As it descends slightly it gains speed, which increases lift over the wing making it climb. This has the effect of it losing some speed, reducing the lift over the wing causing a descent, increasing the speed - and on it goes.
Full scale aircraft from light to airliner can and do experience the same thing. One notable case which comes to mind is the accident sequence of United Airlines 232 which lost all hydraulic pressure and thus had no control surfaces operational. They were controlling pitch (and roll) somewhat using engine power only, and thus did not have exact control of pitch and experienced significant Phugoid cycles.
Side note: You're unlikely to be experiencing ground effect at treetop height unless you have very tiny trees or very large wings.

When an aircraft flies at a ground level approximately at or below the half length of the aircraft's wingspan there occurs, depending on airfoil and aircraft design, an often noticeable ground effect
source

Does this apply to both the situations?

@Jameic that is exactly what I am observing, thank you. I'm certainly to high for Wing in Ground effect I've read that only occurs in roughly 2.3 chord lengths. My longest chord length is perhaps 15 cm the grass in the field is knee height so there is no change of observing that. When I'm more adapt at controlling the thing I'll try a smoother terrain and test WIG out. Is there a difference betwee dead stick oscillation and phugoid cycles or are they the same thing ?

@Abdullah I largely understand the "bobbing", granted my terminology might be off; I'll accept Jameic's answer

As a model airplane pilot I can confirm that this behavior sounds very much like an out of trim model. It sounds to me like it's a little too nose heavy with a lot of up trim. This isn't necessarily a problem if you are new and appreciate the corrective nature of a plane setup this way, but trying to move the center of gravity a bit toward the tail could help tame these tendencies.

@JustWannaFly it can be tough to figure as "a lot of uptrim" will make it stall.  The "soft toss" glide can help.  Nose heavy will go down (ballisticly), then up after it gains speed.  Tail heavy will lose speed and wallow into a stall, flying out more or less straight.  Note this is the opposite of what you see in the "dive test".  Key is to find the glide (trim) speed you like, then move the weight until glide is staticly stable (free flight).  It should nose up slightly with increase in speed.

"You're unlikely to be experiencing ground effect at treetop height unless you have very tiny trees or very large wings." file under Classic  :)

It's worth noting that due to us living in a three dimensional universe, the physical laws do not scale with linear dimension and so the flight dynamics of a drone or model aircraft will still be wildly different than a real aircraft.  In a model plane you might wobble through 2-3 phugoid cycles in a second or two.  In a full sized airliner this will happen on the scale of minutes - it's not nearly as dramatic.

If there ever was a pilot who forgot to throttle down on descent

Full scale pilots trim their elevator for speed, and use their throttle to climb or descend.
"Bobbing or skipping" behavior in cruise is a sign of instability, not surprising for a tailless piece of foam.  If this plane has a published CG location, thrust angle and control surface throws, start there.
If you have a hill nearby, power off glides will help determine if your CG is OK.
If you are nose heavy, the plane will dive to a higher than desired glide speed and require a lot of up trim, but should not constantly pitch up and stall.  Moving weight forward generally improves directional stability.
If you are tail heavy the plane will be more difficult to control and tend to pitch up sharply with speed, sort of what you are seeing.
So work with weight first, in small increments.  You want a slight tendency to pitch up with increased speed in glide so it can self control its speed.  After you get it gliding stably at a speed comfortably above stall...
Make sure your control throws are not too much or too little, then ...
For powered flight, you want your thrust angle slightly down to help counter act the pitch up tendency at higher speeds. Just a few degrees, again adjusting at small increments.  Models tend to be overpowered, this may take a few tries.  The plane should track level or climb a bit when throttle is applied, without excessive trim required (some is normal).
Planes with tails generally are more stable in pitch, but even these will "bob" if they are not set up properly.

At FL-Trees, no power, and a "flat" angle of attack it glides quite stably without any input, picking the nose up makes it "waddle" as it descends "resting" on the flaps rather then the wing. Under power I've seen it has two "stable" modes, the first is a bit of nose up and enough power not to stall, if I can get it "flat" under the same power it starts "ascending" (Which is weirdly pleasing). The undulation occurs with more power. The motor seem just "big enough" taking around three circuits to reach FL-Trees. Thanks for the guidelines though I'll try them out on Friday/Saturday :D.

You have a rear motor mount.  Pushing up in the back makes the nose go down.  But before changing that I would move the CG up a bit.  Check your elevons.  If they are pitching the nose down (with aft CG) it will fly, not as easily.  As you speed up the elevons force the nose down as well, which may be starting your "phugoid" oscillation.  Move the weight up and DO THE GLIDE TEST.  You want the weight forward and the elevons slightly up so it is more stable.  Then try powered flight.  It might be better.  Good luck!

Thanks for all of the help and pointers @Robert DiGiovanni, I included a full image of the model now as well.

@Carel "FL-Trees". What the heck do trees in Florida have to do with anything?? Coffee starts to kick in Oh... wait... that's what you mean!  You get a definite +1 from me for creative writing. Well done!!

@Carmel thanks for the picture.  Your elevons seem a bit up, nose weight and a little less up might be better.  Establish your glide speed, then try power (a little downthrust may help).  A "skippy" weight back plane will be a nightmare to land, anyways.  Maybe get another one, and totally Frankenstein the one you have now, to find what's best.  It will be worth the effort.

@Carel  "The undulation occurs with more power.".  Since you can achieve somewhat stable flight in other conditions, until you add more power, I suspect you may have better stability if you experiment with the downthrust of the motor, changing it by very small increments.  A stable design results from the typical design cycle, which is a spiral of changes.  You change one detail, and several others will need some adjustment, and then repeat the process, and document everything so others can benefit from the final resulting analysis and conclusion.

@Carel  A small aircraft (yours) is flying through a potentially violent air mass for such a small aircraft.  Good analysis and results can be improved by making your aerodynamic surfaces as clean and consistent as possible.  Loose or flappy hinge tape (etc) should be replaced and attached properly.  These small craft can be amazingly forgiving, but do not presume.  Have fun and learn, while also pretending that it is meant to carry precious cargo.  Ensure surfaces are clean and true, for the better experience of discovering the most true complete full defendable and repeatable solution.

@Carel  Adjust motor thrust angle for level flight at full throttle (if that is your goal).  Then observe what happens in a glide.  Or from the other direction -- first adjust for stable glide (no power after powering up to test altitude), then adjust motor thrust angle for stable level cruise at full throttle.  Using tx trim should enable you to alter the characteristics for a full throttle stable climb.  This is assuming you have worked out the best CG.  You should not need to break a sweat keeping a stable design stable in a glide, at its characteristic Vs1.  Have fun!

In my experience with model airplanes the bobbing and dipping motion occurs when the center of lift is forward of the center of mass and represents a series of mini-stalls and recoveries that persist and repeat.
Since simple model planes generally do not have elevator trim, the solution is to move the wing attach point aft in small increments until the plane does not "do the dip". Back in the day, all my free-flight rubber-powered kit planes were set up to make this pitch trim adjustment easy.
If you go too far and move the center of lift too far behind the center of mass, then the plane immediately plunges into a nosedive.

Or add pennies to the nose...

yes yes, like taping nickels to the end of a skipping tone arm!

Moving batteries around a little could be a better use of mass.  Even flipping a battery end for end could be enough change.

The model is of a flying wing so there isn't really a wing attachment point but I'll rotate the battery and get thrifty with the duct tape and pennies. It seems stable when doing a speed of "No thank you gravity" but anything above "Are we there yet ?" results in the skipping; initially I assumed this was a mutually empathetic exuberance for being airborne. Also I'll have to wait until the cold front abates this evening saw me in two trees and a fence.

authors: Bob Jarvis - Слава Україні


text:
Are there any aircraft that either fold or employ folding in their design (such as with origami)?
I became curious about origami and its application to aeroplanes and was wondering if there were aircraft that either fold and/or employ folding in their design in interesting ways.
Let's limit this to mechanisms requiring two or more creases/folds/axes/pivots that alter the geometry of something in a significant way. This should cut out most of the the obvious things e.g. fixtures (doors, food trays, landing gear and stair cases), control surfaces that act alone (canards, rudders, ailerons and flaps) etc. I wouldn't want to rule out any interesting applications however e.g. the use of leading and trailing edge flaps together (2 axes) or swept wings (2 pivots) is certainly acceptable. Finally, anything that is bizarre, curious or weirdly obvious is happily accepted !
Paper aeroplanes should be excluded unless there was some real world application thereof. Space stuff is also out.

Sorry, I don’t mean to rain on your parade, but there’s really no connection between oragami and aircraft construction other than the word “folding”.  The mechanisms involved in folding aircraft wings are not similar in any manner to creases in paper.  You are stretching to establish some commonality that really isn’t there.  Downvoting the question unless you can make a better case for a possible relationship.

No problems, I packed an umbrella ;) It was a bit of a loosey goosey question. I saw an article on a wing design where the wing comprised of two smaller wings placed in "series" and a third extending out from the fuselage would stretch out and join them to form a larger wing with a longer chord. It got me wondering if there were other curious designs. One thought was that one might fold a wing in against the fuselage to form a lifting body. The other was that a wing might be curled up to form a channel wing. I was curious to know if other odd designs had been tried.

I tracked down the plane with the concertina wings it is the NIAI RK-1 and RK-800.

Related question about variable geometries, some examples of which include folding like you describe: https://aviation.stackexchange.com/q/66896/22400

Applying origami is actually a thing in engineering: look at those applications for example. If you are looking for more origami-esque contraptions, you might want to broaden your question to include spacecraft design as well, for which unfolding things is a common design problem.

@null I expressly excluded the space stuff since this is not the site for that and there is a difference between unfurling ones tinfoil device in the vacuum of space versus a "hefty"atmosphere.

Example of folding wings:What is this aircraft with elaborate folding wings?, often used for carrier aircraft. See also Airplane Origami: How Folding Wings Work

@MichaelHall I think you may be lacking in imagination here, they aren't asking if a plane can be made of paper. Origami is a perfectly valid way to brainstorm an idea, just as a small balsa wood model would be.

@zymhan, oh I've got plenty of imagination...  I just don't think the OP did a very good job making a connection between the two concepts.  (He was a good sport about my comment though)  To me it would be like saying "I'm interested in flower arranging and it got me thinking how radio signals 'blossum' out from the antenna.  Can anybody think of other things on airplanes that are like flowers?"

@MichaelHall Ahem... the drone folk use clover leaf aerials ? (Google found this A380 and in the spectrum of bizarre, curious and/or weirdly obvious yet just shy of unicorns but now I'm just egging one on ;D).

@zyman Thanks for the support, I'll get your back in the next sortie ;) Michael was correct in calling me out though, a better question is linked by mins for example (This is quite a duplicate actually). (Glad I posted though, the answers were certainly worth it)

My favourite is probably the Nikitin-Shevchenko IS which could morph between biplane and monoplane by folding its lower wing up into the upper one. The lower wing had two hinges on each side, one at the root and the other at about one-third span, so its outer section was indeed double-hinged.

Does it count to do something similar but cruder and simply let the upper wing go once you have taken off? This slip-wing was trialled on the Hillson Bi-mono before being fitted to a Hurricane.
The Tu-144 airliner and Dassault Mirage "Milan" had retractable "moustache" canard foreplanes which they deployed to improve takeoff and landing performance. Being tailless deltas, they had limited scope for conventional flaps, so put moustaches at the front instead. The moustache of the TU-144 was quite complex, having full-span flaps which amounted to a variable-camber aerofoil, so their trailing sections provide another rare example of genuine double-movement.
The XB-70 Valkyrie folded its wing tips down during Mach 3 cruise, to improve stability and waverider lift. The A-12 variant of the Blackbird was one of a small handful of types to have had a more prosaic ventral fin which folded up for landing and takeoff.
The Wild Goose and Swallow projects of Barnes Wallis at Vickers deserve mention, although they only flew as sub-scale test RPVs. They were examples of what he called the "wing-controlled aerodyne", having swing-wings in place of conventional control and tail surfaces. Not so much a double-action as eliminating the double-action of control surfaces on a swing-wing by using the whole fuselage as one giant multi-functional control surface (a tip he picked up from his work on airships such as the R100).
Does sliding count? The Akaflieg Stuttgart fs29 sailplane has telescoping wings which can pull in to increase cross-country speed. A few other experimental types had them too, such as the Makhonine Mak-10. The Gérin Varivol had a more complex variation on the theme, with a full-span high-aspect-ratio fixed section, which supported leading- and trailing-edge sections that were stowed in the fuselage and scrolled out along the wing like lowering a Venetian blind.
Polyhedra are solids with flat surfaces. Folding them using origami techniques is quite an art form, so I make no apology for the next example. Most polyhedra assemble rigidly but some, known as "breathing" polyhedra, can flex to a limited extent. An example from aviation is the variable-thickness wing experimented with by Rocheville among others, which bulges its top surface upwards to provide high lift for takeoff and landing, then draws it back down to create a thin wing for high-speed flight.

Sure single hinged mechanisms are acceptable as are telescoping wings. Thanks for the interesting examples.

Is there more than one sailplane with telescopic wings? I thought that only the fs29 used those successfully. The SB-11 could slide a wing extension out backwards to increase wing surface area. And if you need more folding vertical surfaces: The MiG-23 and the SAAB 37 Viggen had those, too.

@PeterKämpf Thanks, answer updated accordingly. However I never saw anything fold on a Viggen in flight and I have not discussed folding for ground storage.

www.festo.com has a curious selection of creatures including an Air ray, Flying Fox, Jelly Fish, Air Penguins and a rotating origami ring structure (At least I've seen this folded elsewhere)

@GuyInchbald: Admitted, the Viggen would only fold the vertical during taxi, not flight. If you need more in-flight-folding: There were some involuntary cases, too.

There are many, many examples of folding wing planes. Most, like this Vought F4u Corsair, were designed to take up less space in the tight confines of aircraft carriers or small hangars.

Some planes even fold their wings in more than one place, like this Supermarine Seafire.

Swing wing aircraft like this F-14 Tomcat can sweep forward for slow flight maeuverability and sweep rearward for high-speed flight and stowing.

V-22 Ospreys can even rotate their entire wing and engine assemblies 90 degrees to bring them in line with the body of of the aircraft.

You really should check out this wikipedia link for plenty more examples.
https://en.m.wikipedia.org/wiki/Folding_wing

Thanks, in the original version of the question I meant to exclude the "Folding for storage" variants. They are certainly a valid answer though. I never knew that the Ospreys could crumple up so compactly either.

@Carel: No big deal. I thought it was an interesting question.

Note how the F-14 pops out little epaulette-like leading-edge root extensions when swept back.

@GuyInchbald: I’ve never noticed that before. That’s a great mention.

and the osprey propellers fold too!

Also the B-70 Valkyrie, which had large wing area that folded down for high-speed flight: https://en.wikipedia.org/wiki/North_American_XB-70_Valkyrie  And Boeing's newe 777Xr airliner has folding wingtips.

The Fairey Gannet has a much more obvious Z-fold mechanism than the Seafire.

@GuyInchbald These only exist on the F-14A and only extract at supersonic speeds  http://www.anft.net/f-14/f14-detail-glovevane.htm

@RogerLipscombe that might be the ugliest aircraft I've ever seen. Impressive!

Since

Finally, anything that is bizarre or weirdly obvious is certainly encouraged !

There's the NASA AD-1 with its center-pivot,

its RPV predecessor the Oblique Wing Research Aircraft

and the original Blohm & Voss P 202 WWII design study. Both the Chief Designer Richard Vogt and his Head of Preliminary Design Hans Amtmann were hoovered up by Operation Paperclip and went to work in the US after the war (apologies for the poor image but good, authentic ones are hard to find free of copyright).

Apologies but I added the original design study to your answer.

That long-exposure photo is gorgeous.  Excellent find!

Does anyone know if the handling was adversely affected when the wing is pivoted ?

@Carel there's a flight test results paper here: https://ntrs.nasa.gov/search?q=ad-1&highlight=true&reportNumber=NASA-TP-2223

@OrganicMarble Thanks for the article made for an interesting coffee break :D

When building a lead ballon, the mythbusters employed a origami-inspired folding technique to ensure that the thin material unfolds slowly and without tearing during inflation.

If you're gonna include inflatables, you could say that regular hot air balloons and even blimps employ folding techniques as well.  Perhaps it's less critical when the balloon isn't made from easily-tearable lead foil.

@Darrel Hoffman: Then that gets us into parachutes & parafoils...

@jamesqf I thought about that too, but I'm not sure if a parachute qualifies as an aircraft so much as a landing softener.  Parafoils might qualify, bit of a grey area there.

@DarrelHoffman and jamesqf folding sails, foils and balloons should be acceptable in my view, especially if there are any interesting ones e.g. The origamist (?) Robert Lang worked upon an airbag design; it seems they used the mesh of the 3D model as the basis to fold the bag into the compartment.


text:
When is a CFII required to provide instrument training?
61.195(c) states:

(c)Instrument rating. A flight instructor may conduct instrument
training for the issuance of an instrument rating, a type rating not
limited to VFR, or the instrument training required for commercial
pilot and airline transport pilot certificates if the following
requirements are met:
(1) Except as provided in paragraph (c)(2) of this section, the flight
instructor must hold an instrument rating appropriate to the aircraft
used for the instrument training on his or her flight instructor
certificate, [...]

So as a CFI with a current instrument rating (but not yet a CFII) it appears I can provide instrument training to students working toward their Instrument Rating. I'm trying to find what limitations to this there are - what regs state when training must be from a CFII?
61.65 - Instrument rating requirements (d)(2) states:

(2) Forty hours of actual or simulated instrument time in the areas of
operation listed in paragraph (c) of this section, of which 15 hours
must have been received from an authorized instructor who holds an
instrument-airplane rating, [...]

Throughout 61.65 it consistently states only "authorized instructor" but there is no definition for "authorized instructor" except only as in 61.1(b) which reads:

Authorized instructor means— (i) A person who holds a ground
instructor certificate issued under part 61 of this chapter and is in
compliance with §61.217, when conducting ground training in accordance
with the privileges and limitations of his or her ground instructor
certificate; (ii) A person who holds a flight instructor certificate
issued under part 61 of this chapter and is in compliance with
§61.197, when conducting ground training or flight training in
accordance with the privileges and limitations of his or her flight
instructor certificate; or [...]

61.197 is only Renewal requirements for flight instructor certification.
So by all this, it seems to me that as a CFI with an instrument rating I'm authorized to provide instrument training. There's definitely something missing here and after hours of digging through regs I just can't find any clearer definitions on this. Can anyone help provide some guidance on where it's defined as to when a CFII is required? Thanks in advance.

Welcome to aviation.SE! I believe this question probably answers what you're asking.

No, you cannot provide

instrument training for the issuance of an instrument rating, a type rating not limited to VFR, or the instrument training required for commercial pilot and airline transport pilot certificates

Unless you hold

(1) Except as provided in paragraph (c)(2) of this section, the flight instructor must hold an instrument rating appropriate to the aircraft used for the instrument training on his or her flight instructor certificate,

Having an instrument rating on your CFI makes you a CFII.
You are confusing a CFII, who is a CFI with an instrument rating on his / her flight instructor certificate, with
someone that has an instrument rating on his / her pilot certificate and who also might hold their CFI.
You must be a CFII to give instrument training, the exception to that would be for private pilot candidates from 61.109

(3) 3 hours of flight training in a single-engine airplane on the control and maneuvering of an airplane solely by reference to instruments, including straight and level flight, constant airspeed climbs and descents, turns to a heading, recovery from unusual flight attitudes, radio communications, and the use of navigation systems/facilities and radar services appropriate to instrument flight;

a CFI is able to give those 3 hours of instruction.  Anything beyond private pilot is the realm of the CFII.

I just did show you. In 61.195(c). The question was regarding instrument instruction leading to an instrument rating. If you have a separate question regarding CFI training ask it elsewhere.

Of the 40 hours required of actual or simulated instrument flight by the instrument student only 15 are required to be with a CFII.  See regs published in the question.

@mongo The reg states: 40 hours of simulated or actual instrument TIME. Of that, 15 hours MUST BE WITH A CFII. The other 25 hours could be "hood-time" with a safety pilot.  Get a private pilot buddy and go bore holes in the sky for 25 hours. If you spend 25 hours of hood time or actual IFR with a CFI, it DOES NOT count as instruction. The instrument instruction MUST BE with a CFI-I, I don't know how to make this any clearer. You are setting people up for a FAA violation with your misinformation.

@Bronco6363 I agree with you.  There is a requirement of 15 of instrument instruction with a CFII.  There is no requirement to receive 40 hours of instruction under simulated or actual flight.

The exception to the previous answer would be in the case of someone who is trying to get their Commercial Helicopter rating.  The wording for 61.129(c)(3)(i) is as follows:

Five hours on the control and maneuvering of a helicopter solely by reference to instruments using a view-limiting device including attitude instrument flying, partial panel skills, recovery from unusual flight attitudes, and intercepting and tracking navigational systems. This aeronautical experience may be performed in an aircraft, full flight simulator, flight training device, or an aviation training device;

This leaves this training open to being completed by a CFI, and not requiring a CFII.

As a CFI ie holding a Flight Instructor Certificate with an appropriate category and class rating, you are authorized to provide flight instruction ‘on the control and maneuvering of an aircraft solely by reference to instruments’ (see 61.195(l)).  This means for all dual instrument flight training time going towards either a Sport, Recreational, Private, or Commercial, Pilot Certificate that is not associated with 61.195(3)(c), may be given by a CFI.
Dual instruction flight time specifically required for an instrument rating on either a pilot or flight instructor certificate, for a type rating not solely limited to VFR operations, and an instrument proficiency check, must be conducted by an instructor who holds an instrument rating on their flight instructor certificate, a CFII.
I suspect the primary motivation for doing this by the FAA is because initial flight instruction training only really covers and checks a persons ability to provide primary flight training and said applicant has not yet demonstrated the ability to competently teach instrument instruction to a student pilot.   Given the added complexities of instrument flight, the FAA requires this in order to instruct new instrument students.


text:
Rotor system Weight of a Helicopter
I was wondering whether there is a "rule of thumb" or some kind of estimate (ideally with a source) that states how heavy a rotor system (blades, swash disk, actuators) typically is as a percentage of the total vehicle mass.
"A typical rotor system weighs between X% and Y% of the total helicopter mass."
Something like that would help me a lot! Is there any estimate like that?
Thanks for your help!

With the gearbox it's some 25 to 30% of the empty weight. Unfortunately I don't have a reference though.

Thanks for your comment :) What if the Gearbox is not there? I am thinking of a system that can only do autorotation. Thus, only needs: Rotors, swash-disk, and actuators would suffice, no gearbox is needed.

I'd say 50:50 more or less. Do you have an autogyro in mind?

No, I am looking at landing systems for space craft on planets with an atmosphere and would like to compare autorotation with propulsion (among others). To calculate the mass savings it would be nice to roughly estimate the mass of rotor + swash disk + actuators.... 50:50 you say? I would expect the mass share to get less than 30% not more when the gearbox is left out. (I consider Gearbox and engine to be "payload mass" for science hardware wrt spacecraft)

Let's say that a typical EMS helicopter might have a gearbox of some 250kg and blades+rotorhead+pitch links+swashplate of other 250kg. Do not forget that another planet means another Mach and Reynolds number (due to different atmosphere properties) and obviously different weight (due to different gravity) so those estimations might be wrong

Just out of curiosity: would you use the rotor only for landing?

Here is a link to the NDARC theory manual.  NDARC is a rotorcraft design tool developed by NASA.  The final two or three chapters are a bunch of detailed weight models for rotorcraft.  It should have everything you could possibly want.
Note that if the atmosphere of this other planet has substantially different density, then these equations won't be appropriate.  Higher density means the aero loads are much higher -- this means you can get the same lift from a smaller disk turning more slowly, but it also means the drag (and other) loads are also higher.  In an extreme, think about propellers for air vs. propellers for water.


text:
Could a trained individual passenger escape a commercial plane crash by bringing the necessary equipment to parachute?
We've been through why airlines don't provide parachutes for the passengers. But who says they have to be provided? I want to take my fate into my own hands the next time I fly, by bringing my own parachute. I will train extensively in parachuting beforehand. I'll train in wilderness survival skills, too, and bring aboard lots of survival gear for after I land, including a floatation device in case that's over water. I will at all times keep all this gear on my person ready to go and remain vigilant for any signs of a crash.
Here's what I'd do about each of the points those answers bring up:

"Street clothes will be torn to shreds. Oh, and it's COLD up there." Then I'll wear special Air Force-grade clothing that will keep me warm and won't get torn to shreds, or a pressurized suit if I have to.
"You cannot depressurize an aircraft over 12,000 feet altitude without the passengers passing out rather quickly." Then I'll bring an oxygen supply with me. I don't care about the fates of my fellow passengers. Escaping their fates is precisely why I want to do this.
"It is extremely difficult to exit an unstable aircraft...If the plane is spinning and you are beside the door you might get tossed out and then struck by other parts of the airframe." That's a risk I'm willing to take, a risk that I would prefer over the risk of the crash.
"There are very, very few aircraft accidents that occur with enough time to actually use one." "Almost all fatal accidents happen during take-off or landing, where parachutes would not be of any help." Either assume that I am notified with enough time to escape, or just forget about crashing entirely and consider that I might just decide to leave the plane whenever I want.

The other objections are all about training, which I'll have in spades, or profit, which isn't a concern because I will fund my survival myself.
My questions are:

Is any of the equipment I would need forbidden by TSA rules? If so, is there a country that doesn't forbid that equipment?
How could I escape the plane? This is the one objection that I can't figure out. The only suggestion is to blow the door out with explosives, which are obviously out of the question.

I think that you are actually allowed to do everything you've said in almost any country: you just have to join the relevant Parachute Brigade, they are equipped just like you've described

Do you have a real use case in mind? E.g. tell us about the scenario you are thinking. Most crashes are at landing or take-off (so a para is useless). Collision on air: para is useless. Bomb: para is useless. And you cannot take oxygen: it is too risky: do you want to increase chances to kill yourself or other people just for a less likely case? (and you para could kill you on normal evacuation: much more frequent). Just do not fly if you trust nobody.

Can I ask the OP what precautions they take on the drive to the airport?

Why is this being downvoted without any explanations? How is that helpful to the OP? How is that helpful to SE.Aviation? If it's a bad question, why are there no close votes? If you're going to downvote here, please don't be rude and do it completely absent and kind of justification. It's unfair to silently project our expectations onto each user, perhaps the Q. is in regards to a plot device for a story? In any case, the OP is asking specific, well-formulated questions, and has a long lead-in to make sure we understand the frame of mind for the Q.

Probably, @KennSebesta, because every single point the OP wants to ignore was addressed in the question he linked to. Every single point was well discussed in said linked question, and every single point he wants to ignore is being addressed again in the answers here. Basically this question boils down to, "Ignoring all the practical, real-world reasons what I'm asking can't be done, can I do this?". Maybe this would be better suited at [scifi.se] or [worldbuilding.se], but not here where reality rules...

@FreeMan thanks for voicing that criticism. It sounds fair and a valuable point for discussion.

Test pilots of commercial aircraft wear parachutes, so they seem to think it can be better to use a parachute than to crash with the plane in case of failure. Link borrowed from this question.

This is sorta interesting, but simply due to the brash wording of the question, I voted down. Thsi might be viable as "I'm writing a novel, and need to verify plausibility of...". As it stands now, this just irritates me.

@Jpe61: I understand your reaction, I may be wrong but I've a different feeling: The OP is saying "I read valid reasons explaining why jumping is not seen, but I'd like to know whether in theory this is possible, or there are fundamental showstoppers". The result is a long question, which might be perceived as you did, but alas, without these carefully chosen words, the answers will be the ones the OP already read and/or the possibly of closing with the stupid "duplicate" is high. I don't see how to rephrase the question without allowing such useless repetitions or closure.

It would be very easy to phrase the question with far less bravado.

I agree with @sophit, you'll have to join the parachute brigade. Otherwise, as a civilian, this is not practical, feasible, and allowed. In the case of an emergency, the pilots will certainly not let you open the doors. You won't have time to strap up anyway. And it'll be very awkward if you wear the parachute when you feel something is wrong. Also, you'll never get past airport security, because although your intentions are not, it looks very alike some hijacker who plans to escape via para when things go haywire.

Well, it depends. Remember, first off, that most aircraft accidents take place during takeoff and landing, at altitudes were a parachute is going to be moot and not very useful for survival.
Second, as a passenger, you would have to be aware of the threat and have enough time available to realize you had to abandon the airplane, then do so with enough time that you could get out and deploy a parachute safely safely.
Third, we would have to consider the case where you were attempt to abandon the aircraft in a situation, where it’s not necessary, causes additional damage to the airframe that precipitate to crash. For instance, you open an emergency exit door in flight, and that door is torn off by wind forces, and strikes the air frame and damages it in a way that the airplane is no longer controllable, you have now caused the airplane to be incapable of continued flight, with a high risk of crashing, and you’re now liable for criminal manslaughter charges should your actions results on the death of someone.
Fourth, a catastrophic threat to the aircraft may not be known to you, the passenger, until it’s all over.  Bailing out of an airplane as you suggest, is really only useful in a very slim number of accidents, chiefly ones that occur with enough altitude to reasonably assess the danger that you were in, then make a commitment and make your way to an exit to escape the aircraft. It’s possible a passenger bail out scenario might have been useful in the case of Cactus 1549 (Sully’s bird strike/ forced water landing in the Hudson), or possibly the two accidents involving the 737 MAX, which grounded it.  It will not, however, save you from an incident like what happened at Tenerife in 1977.
Finally, your risk of dying in an airplane crash, while flying aboard a scheduled air carrier over international waters or a country with a stable government in place is so low, it’s not even worth worrying about. You have a greater chance of drowning in your own bathtub or choking to death on your dinner tonight. There are a hell of a lot of other things that could kill you, and are far more likely to do so, than a commercial aircraft accident.

And of yourself you’re orders of magnitude more likely to get killed in a training accident learning to parachute than you are on a commercial flight.

I’d go further than that and say that your most optimistic chance of survival parachuting from a commercial jet is worse than your chances of surviving a crash.

@Frog: "your most optimistic chance of survival parachuting from a commercial jet is worse than your chances of surviving a crash", test pilots have parachutes when testing commercial aircraft. There is a procedure, and exits are prepared, but that's another point, which can be explained to the OP anyway.

@mins they do, but I’m not aware of a single instance of one being deployed, successfully or otherwise.

Pilots and film crews have exited a plane before a planned crash test https://en.wikipedia.org/wiki/2012_Boeing_727_crash_experiment

Yeah, but that was very tightly controlled case where the situation was set up that a bail out was possible from a perfectly good airplane. Comparing that with attempting to abandon a disabled airplane in an emergency is like comparing apples to oranges.

So, if you could even open the doors, which you can't, you'd need a parachute, oxygen supply and mask, survival gear, and probably thermal clothing.
That's a problem. The parachute would likely be legal as carry on luggage, the thermal clothing as well. Both would however make people really really look at you with suspicion about your reasons for being on the flight.
The oxygen supply is even worse, it's a pressurised cannister which is definitely prohibited.
The survival gear would almost certainly include a knife and other sharp implements, prohibited. As well as bottled water and/or food paste, and water purification tablets (and possibly other chemicals) which would either be prohibited or draw a lot of attention to you at security.
And that's without taking into consideration that all that stuff is going to be rather bulky and heavy, quite possibly putting you way over the carry on luggage restrictions. And making egress (if you could even get the door open) very problematic.

If O2 is necessary for the jump, then an O2 concentrator might be a good place to start. Some are certified to 18k', and with a proper facemask have been demonstrated to go to 24k' and beyond.

Can you get one past TSA, @KennSebesta?

@FreeMan yes. They're medical equipment and are pretty standard for people who need supplemental O2. They scavenge O2 from the air by stripping N2 from the free gas, thereby raising the partial pressure significantly. As a result, there is no meaningful compressed O2 stored in the device, and it doesn't present a danger.

Fair point, @KennSebesta. Hadn't really thought that through. My bad.

Either assume that I am notified with enough time to escape, or just forget about crashing entirely and consider that I might just decide to leave the plane whenever I want.

You can't open the doors at altitude.
Time - Unless you are already wearing the parachute, you'll not have enough time to get it from where it is, and put it on.

You can't open the doors at altitude *anymore. Source: Cooper, D.B.

@tedder42 *You can't open doors you can safely jump out of at altitude anymore. The Cooper Vane was for the specific airstair configuration. Regular aircraft doors did, and still do, depend on pressurization. Maybe the exception is the 737NG overwings.


text:
Where can FAA Letters of Interpretation be found?
The FAA issues Letters of Interpretation and Chief Counsel Opinions in order to clarify existing rules and regulations.  Where these can be found and searched when looking for clarification of a rule?

The FAA Regulations Division maintains a webpage which includes a searchable database of "Legal Interpretations & Chief Counsel's Opinions".  It includes the following disclaimer:

Legal Interpretations and the Chief Counsel's opinions are now
  available at this site.  Please note that not all interpretations or
  Chief Counsel's opinions are available at this time.  This database
  consists of legal interpretations issued from 1990 to the present and
  will be updated on a regular basis.

The letters can be searched by year and/or keyword.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
What are the legal requirements to be a "crewmember"?
14 CFR 1.1 includes the following definitions related to crewmembers:

Crewmember means a person assigned to perform duty in an aircraft during flight time.
Flightcrew member means a pilot, flight engineer, or flight navigator assigned to duty in an aircraft during flight time.

Some of the regulations (although not all instances) also refer to a required crewmember.
Anecdotaly I have heard of people being assigned as a crewmember on the spot so that they could fly in an aircraft that is limited to "crewmembers only" (i.e. some world war two era aircraft, etc.) by saying something like:

"If the normal gear extension fails during this flight, then you will be responsible to pull this emergency handle over here.  Okay, got it?"

I'm not sure whether or not that would technically qualify someone as a crewmember, so are there any legal interpretations from the FAA or any guidance on what actually does qualify someone as a crewmember?

I wouldn't be surprised if the emergency gear guy is considered a crewmember. Jumpseaters are considered additional crewmembers and they are basically just another set of eyes and ears.

@casey Yeah, I've heard of a few different operators doing similar things.  I'm just trying to track down what is technically required by the FAA in order to be considered a crewmember.  :)

Your example is probably a military thing rather than a flight regulation thing: i.e. the military forbids passengers on some flight, so an arbitrary duty is found for a person who wants to fly. The real crew would probably be in trouble if it came to light.

I'll take a stab at this one, though I can't guarantee the correctness of my interpretation (the regs don't seem exactly clear on the subject - at least not to me, so there's a lot more "inference" here than I'd normally be comfortable with).
In most cases under Part 91 the only requirement to be a "crewmember" is that the pilot in command says you're a crewmember -- i.e. the PIC determines that a person is qualified to perform a specific duty and designates them to perform it.
In the absence of anything more specific this is the only requirement I can turn up (and it's an inferred requirement - I didn't find anything specifically stating this, but 91.533 regarding flight attendants strongly implies that it's the correct interpretation. There's also a counsel opinion from 2008 - the Schehr letter which talks a little about who is a "crewmember" for purposes of riding in jumpseats that are placarded "For Crew Use Only". That interpretation is based on Part 135, but could be logically extended to Part 91 (and possibly some 121) operations too).

There are frequently more specific requirements though, particularly for positions that are important to flight operations. For example if you buy yourself a nice shiny Lockheed Electra to duplicate Amelia Earhart's trip under Part 91 rules you still cannot simply designate someone off the street to be the Flight Navigator: the FAA has specific requirements for Flight Navigator certification, and someone acting as the flight navigator would need to meet those requirements.
There are also more stringent requirements that are imposed for Part 121 and 135 operators. For example they can't simply designate a person as a flight attendant based on the PIC's determination that they are qualified: There are requirements for filling that crew position which must be met (FAR 121.421 & FAR 135.295 respectively, and possibly other regulations).

Thanks, I appreciate the research.  However, my question is about a general crew member, and not the requirements for a flight attendant or flight navigator.  91.533 outlines what the PIC must confirm in order to determine that they are qualified as a flight attendant, but what about "other than flight attendants" and non-required crew members?

@Lnafziger by extension from 91.533 (at least under part 91, and on simple aircraft) the broad standard is "The PIC finds you qualified to perform , and assigns you to perform that duty" unless a more specific regulation applies (so the "gear guy" in your example is, as best I can tell, a legitimate crewmember - though the restriction's spirit is probably required crewmembers). "More specific regulations" seem to apply in most practical cases though (e.g. even part 91 Fractional Ownership ops have to deal with 91.1083 for anyone they want to designate as a "crewmember").

Hmm, I read that as "The PIC find you qualified to perform flight attendant duties, and assigns you to perform that duty."  I don't see anything that would allow it to be extended to some other duty....

@Lnafziger Digging back from the counsel interpretation I just added to the answer someone be found qualified & assigned to perform "server" duties (feeding people, but not any of the flight attendant safety stuff), a mechanic can be a crewmember (presumably if diagnosing engine faults), etc. - At least under Part 135 designating someone as a crewmember triggers other requirements though (like "[their] duties and responsibilities must be included in the air carrier's general operations manual." - presumably to preclude designating a random  person off the street as "the gear guy")

Yeah, I just finished reading the interpretation that you just added and it is pretty much exactly what I was looking for, thanks!

I'm not aware of a glossary anywhere that defines all the possible types of required crew, but I can name some.
The important thing is that the definition of "required crew" varies based on the requirements of the flight, not the certification of the aircraft.  Required crew are also those crew required to safely operate the aircraft under the conditions of the flight.  For instance, a photographer (who has no other duties) is not "required" crew even if the purpose of the flight is to conduct photography.
First you have the crew required by FAA regulations:
Obviously, the pilot, and co-pilot if required.  Along the same lines, an instructor, safety pilot, or examiner would also be considered required crew if required by the flight (if the pilot is not rated in the aircraft, or is wearing a view limiter, etc.)  Also, relief pilots, if the duration of the flight is long enough.
Navigators and flight engineers, although few aircraft still need engineers and navigators are basically obsolete now.
Flight attendants, if the aircraft carries more than nine passengers.
If the aircraft is carrying hazardous materials, then a cargo specialist might be required.
Where the definition of "required" and "crew" gets a little sloppy is when it comes to experimental aircraft.  If the aircraft is something like a new airliner prototype, then the engineers will have produced a mountain of paperwork detailing exactly what the crewmembers are and what they need to do.  But for amateur-built or ex-military aircraft, "required" is a bit more nebulous.  One particular case is the Commemorative Air Force's B-29, which the CAF likes to fly with extra engineering crew that the USAF didn't use when they operated it:
http://www.avweb.com/news/pelican/182150-1.html?redirected=1
The FAA isn't always thrilled about this, even where there's an engineering justification for it.  An ad-hoc "emergency gear lever crewman" on what amounts to a pleasure ride would be one of those wink-and-nudge cases that, if the flight actually experienced a problem, would probably incur some ill will from the FAA or NTSB investigators.
Anecdotally, I've also heard of amateur-built aircraft taking up two crew on testing flights, one to fly the plane and another as, essentially, a flight engineer, especially if the engine being used is not an ordinary Lycoming/Continental.
In the US, regulations restrict the use of the jump seats only to approved persons, such as a relief pilot, company check pilot, or someone else who "belongs there."

Thanks for the answer, but my question is specifically about the term "crewmember" and not "required crewmember".  For this particular term, I don't care if they are required or not.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Are fireworks a hazard to aircraft?
Tonight's fireworks got me wondering:  With all of the firework displays happening at the same time, how do they keep aircraft away from them?  Is there any risk of collision or blinding the pilots?  Is there anything that pilots need to do differently when fireworks are in the area?

A brief Google search suggests that commercial fireworks may peak on altitude at about 1000'. This is hardly a scientific response of course (hence a comment rather than answer), but it would seem the risk of collision is low against a safely operating pilot that takes this into account for altitude selecion. I don't know what distance makes blinding a factor, but I suspect night vision loss is a real risk.

@mah Aircraft are still below 1000' for takeoff and landing, and on major holidays in very populated areas, fireworks are everywhere!

I haven't seen many commercial fireworks displays off the departure end of an active runway.

@DanPichelman Haha, there are a lot of smaller airports and the airplanes there are typically flying a lot lower in the traffic pattern. While I'm sure that the big firework displays aren't being launched from the middle of the runway, are they close enough to create a problem for a pilot?

@DanPichelman I saw a bunch of them last night well inside the Class D ring of my home field - Somehow I doubt anyone notified the tower they were putting on a display (but hey they close the tower at 11 - maybe they called & nobody answered?  :)

Since this question is not country specific, I would like to mention that fireworks of pretty much any kind can be used at any time in India legally. The only laws that restrict fireworks apply to decibel levels of said fireworks.

In some countries it's illegal to launch fireworks in the vicinity of airports. I always thought this was the case in the US as well, perhaps I'm mistaken.

@roe I'm sure that it is, but that doesn't always so people!

I would venture that not many people are firing off commercial-grade fireworks. And those that are have already run through the restrictions of firing them around aircraft operations. Any consumer-grade fireworks aren't very likely able to reach any safe-flying aircraft. And if someone were intentionally launching them off near an approach or departure end of a runway (despite the quick potential for a visit from the authorities), I'd guess they'd be pretty unlikely to be large enough to seriously harm an aircraft.

@falstro - It's not illegal to shoot off fireworks from an airport. It's illegal to do so without anyone at the airport knowing what you're doing. There's an annual air show and fireworks display at Addison Airport; the fireworks simply occur after all the aircraft are on the ground, and there's a TFR for non-participating aircraft most of the day anyway.

@Shawn - I think I have to disagree; roadside fireworks stands have some pretty big boomers behind the counter if you have the cash. Still smaller than professional-grade, but I still think the consumer-grade ones could punch a fist-sized hole in a small craft's wing, if you were unlucky enough to get a direct hit from some yahoo with more money than sense launching them along the traffic pattern of your home strip.

@KeithS - I still say it would be a serious idiot shooting off fireworks like that at any aircraft low enough to be hit, and unless they were right off the end of the runway (again, insuring a pretty quick visit by authorities), they'd not be likely to hit an aircraft. Even commercial grade fireworks don't fly much more than a few hundred feet off the ground. Maybe 1K ft for the really big ones. But even though an aircraft skin is likely aluminum, I doubt very many fireworks would hit hard enough to punch through. At the worst, it would be very distracting to the pilot on takeoff or landing.

@KeithS - That said, I've actually been shot at with a rifle while doing simulated engine failures in training. A couple of times. A bullet round will most definitely punch a hole in your airplane. And, unless it hits the cockpit area, you likely won't even notice until you land and see the hole. :-/  And yes, those occasions did result in reports to the authorities about the farmers that shot at me.

In canada and I assume in a similar way in the United States, fireworks are treated like any other obstacle such as a communication tower, building, or crane. The location and elevation of the firework activity is assessed for impacts to any instrument procedures, airways, minimum altitude areas, etc. and NOTAM to temporarily amend affected procedures will be issued prior to the activity to advertise the changes and their effective periods.
Having said that, this doesn't address the VFR side of things. Operating near fireworks is essentially the same see and be seen idea. NOTAM get issued here all the time for the fireworks activity themselves and they would include a radius from the centre point. Simply avoiding the area is usually sufficient.

This is what usually happens in the US as well - but an important caveat is that it generally only happens for organized displays. When somebody decides to launch some bottle rockets from their back yard there is usually no notice (especially if there's no permit & the display is technically illegal) - it's basically "big sky theory" & the generally low likelihood of a plane and a mortar shell being in the same place at the same time...

In addition to the previous answer, there are often TFRs (Temporary Flight Restrictions) around the areas of organized fireworks displays- before you fly you're responsible for knowing where these locations are and not flying into that airspace.
As far as avoiding the stray bottle rocket, hopefully you're high enough in cruise and lucky enough in takeoff/landing to avoid these hazards.
As far as blinding, the fireworks can take away your night vision if you're close enough but I don't think it would blind. At least it has never happened to me.

Most firework displays peak at about 200 feet above the launching site. Any aircraft low enough to tangle with fireworks is too low anyway.
Pilots see fireworks as little flowers on the ground. They are not big or impressive from the air.
Also, the individual stars, and even the shells, do not weigh much. Unless a firework went into a jet engine, it could not damage an aircraft.

Welcome to Aviation.SE. Aircrafts are easily below 200 feet for takeoff and landing, this might suggest that special regulations exists for fireworks near airfields. For your last assertion, do you have any data or source?


text:
What is the legal interpretation of rest for FAA Part 135 operations?
14 CFR 135, Subpart F contains the rest requirements for Part 135 operations.
The rest requirement for 1 and 2 pilot unscheduled crews (typical) comes from:

135.267(d)
  Each assignment under paragraph (b) of this section must provide for at least 10 consecutive hours of rest during the 24-hour
  period that precedes the planned completion time of the assignment.

What does the FAA consider rest and what actions by the company will interrupt the required rest?

The 10 hours of rest need to be uninterrupted, planned, and not spent on call. Essentially, the pilots need to have 10 hours in which they are not obligated to do any work for the company at all.  Note that it does not mean "10 hours of sleep", and it is up to the pilot what they do with their time off.  (They can even technically fly for another 135 operator during that time, but they run the risk of being "careless and reckless" if they aren't well rested.)
NATA has an article about some of the FAA's legal interpretations (pdf) that's pretty interesting and relatively recent (2011). Their paraphrase of the FAA's varying interpretations is this:

the FAA has said that for a “rest period” to be legal it must be: 1) continuous, 2) determined prospectively (i.e. known in advance) and 3) free from all restraint from the certificate holder, including freedom from work or freedom from the present responsibility for work should the occasion arise.  Being on call is specifically mentioned as not rest.

This is an interpretation that is repeated throughout a lot of articles and forum discussions. It's also often pointed out that many (most?) 135 carriers blatantly ignore these rules, as most charter operations require pilots to be on call.
For example
You arrive at 19:00, and are done with paperwork / postflight / etc by 19:30 and leave the airport. The moment that you leave the airport is the beginning of your rest period; it will need to last 10 uninterrupted hours, which means until at least 0530 the next morning (1930 to 0530) to mean anything.
If you get a call from dispatch at 21:00 telling you about a flight that might take place in the morning and that you must be available for a call at 05:00 just in case - your rest ends at 05:00. Because your rest period ended 30 minutes early (when you were required to be available), you did not get the 10 hours of rest and can't take a flight without violating 135.267(d).
Another caveat:
There are no explicit limits on duty time (the exception being 135.267(c) involving a "regularly assigned duty period of no more than 14 hours" which doesn't apply to most 135 operators).  The limit is on planned flying conducted under 135.
As long as you plan on parking the airplane when you can show 10 hours of rest in the previous 24 hours (which is why some people incorrectly assume that you are limited to 14 hours of duty: 24 - 10 = 14!), you can be kept on duty afterwards.  You can even fly under Part 91 for the company.  You can clean the airplane, or do office work.  While none of that is rest and it is still duty, it is legal unless you get in the airplane to fly under Part 135 without getting the required rest.

It doesn't matter if you have to drive, they can assume you live on the airport and rest officially ends when you duty-in.  I fondly recall my early-morning hotel van rides to the airport while still "resting".

Shouldn't the call at 21:00 count as interruption anyway (so even if you were originally planned to start later and they told you to start 5:30, it would still not be legal)?

@JanHudec It only interrupts rest if you are required to be available for the call.  If they call, and you choose to answer, it does not interrupt the rest period.  Keep in mind that rest != sleep, just time that you aren't required to do anything for the company.

"most charter operations require pilots to be on call." Could you give us more details as to what you mean by that? For how long do they have to be on call?  I suspect there is some subtlety being missed here.